The invention relates to an apparatus for processing input video signals having the form of digitally encoded samples and being arranged so that successive samples produce a television picture on a display screen by line and field sequential scanning. The apparatus comprises: a field store; a first address generator for generating a set of addresses for addressing memory locations in the field store, the set of addresses being generated from input signals defining a given geometrical modification of the input television picture represented by the input video signal samples; an interpolator for receiving said input video signals and producing interpolated samples in response to said input signals defining the given geometrical modification, said first address generator being effective to assign to each sample produced by the interpolator, a memory location indicated by an address of said set of addresses; and a second address generator for generating addresses of at least a selected part of the field store to enable video signal samples to be read from the locations of the selected part of the field store to form a series of video signal samples which represent the modified television picture.
Such apparatus is disclosed in U.K. patent application No. 2119197A, where the given geometrical modification is a rotation of the television picture.
If the apparatus is used to process video signal samples representing a television picture constructed from two interlaced fields and a geometrical modification is produced which involves an input line intersecting two or more output lines, then some of the sample locations on the intersecting input lines of input field 1 will correspond to sample locations on lines of output field 2, and correspondingly, some of the sample locations on the intersecting input lines of input field 2 will correspond to sample locations on output lines of field 1. Thus, if full resolution is to be obtained, there will be a mixing of fields 1 and 2 in the output television picture. While this is satisfactory for unchanging picture information when the contents of fields 1 and 2 are identical, if there is any movement in the picture, the information content of fields 1 and 2 will be different and mixing the fields is then undesirable. One solution to this problem is to use only field 1 or field 2 information and to store that field twice. However this has the disadvantage of losing half the available input information.